Our prior investigations in normal rhesus monkeys, using a sensitive and specific radioimmunoassay, demonstrated that maternal serum AFP levels were first detectable at 6-8 weeks followed by a rapid rise and plateau at 100-200 mg/ml. A slow decrease occurred during the last 3 weeks of gestation. This data has been confirmed in additional pregnancies. However in several instances of fetal distress or death maternal serum AFP levels were not significantly elevated above the expected normal ranges. Although fetal death was not associated with abnormal elevations of maternal AFP, an experimental model in which the umbilical cord was ligated at 70 days showed a 40 fold increase in maternal serum AFP within 24 hours. This level was maintained for about three weeks until fetectomy after which time there was a slow decline to normal. BIBLIOGRAPHIC REFERENCE: Tullner, W., Starkovsky, N., Hodgen, G., and Cohen, H.: Alpha-fetoprotein in Subhuman Primates: Factors Affecting Serum Levels During Pregnancy. In: Protides of the Biological Fluids, 24th Colloquium, Bruges, 1975, Ed. H. Peeters, Pregamon Press, Oxford, 1976.